In this study, we present the inverse analysis for identification of hammering signal in non-destructive hammering test. As the observation value, the observed voltage from microphone is transformed to sound pressure level, and the performance function is defined by square sum of residual between the obtained and the computed sound pressure levels. Here, the problem is to find the input sound pressure level so as to minimize the performance function. The formulation for this problem is carried out by the adjoint variable method, and the numerical simulation of the sound pressure level propagation is carried out based on the wave equation and the finite element method.